Abstract: Insulation failure can cause motor turn-to-turn，phase-to-ground and ground-to-ground faults，ranging from equipment downtime，or endangering personal safety，resulting in catastrophic consequences. In order to be able to carry out online sensitive monitoring of the deterioration state in the early stage of turn insulation deterioration，a new method for fault diagnosis of the turn insulation of the inverter drive motor with multi-frequency characteristics based on switching oscillation was proposed. First，the signal was extracted by the high-frequency voltage and current sensor. Then，it judged whether it is a switching transient process，if so，the peak current amplitude under the switching transient and bandpass filters were used to process signals in multiple bands，and it calculated the average value of the peak point of current amplitude in a cycle，and selected insulation fault sensitive frequency by the result. Finally，the average value of the current peak point under the switching transient under different operating conditions and the scatter plot of the current amplitude under the switching event were drawn. According to the experimental results，when the insulation is degraded，the fault diagnosis of the turn insulation can be performed through the switching oscillating current signal in the multi-frequency.

      Key words: insulation deterioration；switching oscillation；multi-frequency；fault diagnosis